ESTIMATION OF TRUSSIC ACID 177 



again dry. The test papers, which may be kept in stoppered 

 bottles for some time without deterioration, turn an orange 

 red in the presence of fumes of hydrocyanic acid. The test 

 is very delicate, and the rapidity of the change in colour de- 

 pends on the amount of prussic acid present, so that if the 

 quantity be very small the paper may have to be suspended 

 in the test tube containing the material to be tested, for a day. 



This test has been modified by Waller so as to give quanti- 

 tative results, but it has been pointed out by Chapman * that 

 the coloration is due to reduction, and is, therefore, not speci- 

 fic for hydrocyanic acid ; accordingly the method must be 

 used with caution. 



It was found that if a leaf of the cherry laurel. Primus 

 Laurocerasus^ be immersed in an aqueous solution containing 

 •05 per cent picric acid and 5 per cent sodium carbonate, the 

 leaf is unharmed and the fluid undergoes no obvious change. If, 

 however, the leaf be immersed in the same fluid to which chloro- 

 form has been added in the proportion of "4 c.c. per 100 c.c. of 

 fluid, the formation of hydrocyanic acid takes place and the 

 sodium picrate turns red. The intensity of the colour is the 

 basis of the quantitative estimation of the hydrocyanic acid. 



The standard colour is obtained by mixing together equal 

 volumes of the picrate fluid and -002 per cent hydrocyanic 

 acid. This mixture, which contains 10 mgs. of hydrocyanic 

 acid per litre, is allowed to stand for twenty-four hours in an 

 incubator kept at 40° C. The intensity of the colour is 

 designated by the symbol Tio, and corresponds to 10 mgs. 

 of hydrocyanic acid per litre ; a colour intensity of Ti similarly 

 corresponds to i mg. of hydrocyanic acid per litre. By diluting 

 the standard solution, a gamut of colour intensities may be 

 obtained Ti, T2, T3 . . . the figure in each case correspond- 

 ing to the number of milligrams of hydrocyanic acid in one 

 litre of fluid. These colours may be matched closely by 

 aqueous solutions of potassium bichromate. 



In making up the standard solution it is important to 

 allow the mixture to stand in an incubator kept at a tempera- 

 ture of 40° C. for certainly not less than an hour owing to the 

 slowness of development of the full tint. 



* Chapman : " Analyst," igio, 35, 469. 

 12 



